Warn on missing pkg-config, plus trivial comparator typo fix.

Change-Id: I68eef93ee70a934feb9a53cca050b81a4e66e26c
BUG: 921817
Signed-off-by: Justin Clift <jclift@redhat.com>
Reviewed-on: http://review.gluster.org/4672
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>